1. Field of the Invention
The present invention relates to an optical transmitter module suitable for use in optical fiber communications and equipped with an electricabsorption type optical modulator for converting an electric signal to a light signal, and particularly to a circuit configuration of an optical transmitter module suitable for use in optical communications, which requires a satisfactory high frequency characteristic.
2. Description of the Related Art
As a prior art of a laser diode module equipped with an electricabsorption type optical modulator, such a configuration as shown in FIG. 10A has been disclosed in Japanese Published Unexamined Patent Application No. Hei 9-252164. In a hermetic package having an electric signal input terminal, there are provided a micro-strip line 3 connected to its corresponding signal pin 12 of the hermetic package 8, a damping resistor (Rd) 2B having one end electrically connected to the micro-strip line 3 and the other end electrically connected to a terminal resistor (Rt) 2A, a modulator unit (hereinafter called xe2x80x9coptical modulatorxe2x80x9d (MD)) 1 of a laser diode with a monolithically integrated electricabsorption optical modulator, which is electrically connected in parallel to the terminal resistor 2A, and an optical system for coupling the output of the optical modulator 1 to an optical fiber 9. These circuit configurations are formed over a sub-carrier 11 comprised of an insulator such as AlN. The sub-carrier 11 is further fixed to a carrier 6 and electrically grounded. Furthermore, a photodiode for optical power control 5 is fixed onto the carrier 6. A Peltier element used for cooling and a temperature monitoring thermistor 7 are also provided within the hermetic package.
FIG. 10B is a top view showing the sub-carrier 11 in a developed form. The laser diode 1 is fixed onto a grounding electrode pattern 13. The strip line 3, the terminal resistor 2A and the damping resistor 2B are also formed by evaporating a metal thin film onto the sub-carrier.
In the prior art, the optical modulator 1 is defined as being capable of being described by a capacitance component alone, and the impedance of the optical modulator 1 is reduced with respect to a high frequency input electric signal. Therefore, the damping resistor 2B is inserted into the package to thereby reduce impedance mismatching at a high frequency and lessen a return loss. The reduction in impedance mismatching at the high frequency is achieved even by connecting the damping resistor (Rd) in parallel with the terminal resistor (Rt) as in the case of a configuration example shown in FIG. 12 (circuit example shown in FIG. 13).
Originally, lessening the return loss in the input electric signal of the optical transmitter module is a technique extremely important for the purpose of accurately converting the waveform of the input electric signal to the waveform of a light signal. It is therefore necessary to accurately describe an equivalent circuit of the optical transmitter module including the optical modulator 1. The known reference referred to above discloses that the optical modulator 1 can be described by the capacitance component alone as mentioned above. However, the equivalent circuit of the optical modulator 1 cannot be essentially described by capacitance alone. It is necessary to describe a photo-carrier generated upon light absorption in the form of an equivalent circuit.
FIG. 5A is an equivalent circuit described in consideration of a photo-carrier generated upon light absorption. As shown in the same drawing, the equivalent circuit can be described by connecting a voltage depend current source in parallel with a capacitor. Further, the voltage depend current source can approximately be replaced by a resistor as shown in FIG. 5B. At this time, the amount of a reduction in the impedance of the modulator unit changes according to the magnitude of an amount-of-change ratio (=1/Rph) of a current bearing a photo-carrier to a voltage applied across the modulator. When the intensity of light inputted to the modulator actually increases, the present ratio, i.e., Rph becomes small, thereby leading to a large reduction in impedance. Such a reduction in impedance due to the photo-current becomes pronounced from a relatively low frequency domain. Therefore, the prior art is accompanied by a problem that the return loss increases due to the above-described impedance mismatching from the low frequency, so that the waveform of the electric signal is not converted to the waveform of the light signal with fidelity.
Further, the characteristic of a response to a light signal from an electric signal greatly varies within a band width in such a conventional configuration as shown in FIG. 11. This is because resonance occurs due to a capacitance component included in an optical modulator and an inductance component included in a wire and hence peaking occurs in response as shown in FIG. 6. The horizontal axis of FIG. 6 indicates the frequency of an input electric signal and the vertical axis thereof indicates an optical output response, respectively. Curves in the same drawing, each of which is indicative of the relationship between the input electric signal and the light output signal, differ in shape according to the intensity of light inputted to the optical modulator as in the case of three curves illustrated in the same drawing, for example. Further, A in the same drawing indicates a deviation in band width. The deviation in band width means the rate of change in optical output response within a required band width. The more the deviation in band width becomes large, the more distortion of the waveform of the light output signal increases. FIG. 7 shows an example of a waveform of a light signal outputted from a module having such a large deviation in band width. The present example shows the result of simulation at the time that an ideal rectangular wave is inputted as an input electric signal. The horizontal axis in FIG. 7 indicates time, and the vertical axis indicates the intensity of light, respectively. If an optical output response to the frequency of the input electric signal is constant, then the input rectangular wave is to be outputted as a light signal as it is. However, projection like distortion occurs in the output light signal according to the deviation in band width in FIG. 6 as is understood from FIG. 7. Further, the projection like distortion is large as the deviation in band width increases. Thus, the prior art is accompanied by a problem that a satisfactory light-signal waveform cannot be obtained with an increase in the deviation in band width.
An object of the present invention is to implement an optical transmitter module which solves the foregoing problems and is equipped with an optical modulator less reduced in impedance, which converts the waveform of an input electric signal to the waveform of an output light signal with fidelity, and by extension an optical transmitter module having a satisfactory high frequency characteristic, wherein even if the power of light inputted to an optical modulator changes, the waveform of a light signal is not distorted, i.e., the characteristic of a response to the light signal is not degraded.
Another object of the present invention is to implement an optical transmitter module which solves the above-described problems and is equipped with an optical modulator which provides a small deviation in band width and is hard to develop peaking in response, and which is capable of obtaining a satisfactory light-signal waveform.
In order to achieve the above objects, the present invention principally comprises an optical transmitter module which comprises an electricabsorption type optical modulator for modulating a light signal in response to an electric signal, a first resistor having one end connected to the optical modulator and the other end grounded, and a second resistor having one end connected to an input supplied with the electric signal and the other end connected to the optical modulator and the first resistor respectively, and wherein the second resistor and the optical modulator are connected to each other through a first inductance.
The optical transmitter module may include a high frequency line connected to the second resistor and for transferring the electric signal to the optical modulator.
The optical modulator and the first resistor may be connected to each other through a second inductance different from the first inductance. The optical modulator may be integrated into a semiconductor laser diode.
At least the optical modulator, the first resistor, the second resistor and the first inductance may be held in one package. In that case, the electric signal may be supplied from outside the package. Alternatively, the electric signal may be generated inside the package.
The value of the second resistor may range from over 3 xcexa9 to under 25 xcexa9. Alternatively, the value of the first inductance may be set greater than or equal to 0.1 nH. Alternatively, the distance between the second resistor and the optical modulator may be set so as to be less than or equal to 7.5 mm.
The optical transmitter module is equipped with a first substrate, and a second substrate electrically isolated from the first substrate. The optical transmitter module may take a configuration wherein at least the optical modulator and the first resistor are provided over the first substrate, and at least the second resistor is provided over the second substrate.
The optical transmitter module is equipped with a first substrate, and a second substrate electrically isolated from the first substrate. The optical transmitter module may take a configuration wherein at least the optical modulator and the first resistor are provided over the first substrate, and at least the high frequency line and the second resistor are provided over the second substrate.
The first resistor and the second resistor may be placed on the sides opposite to each other with the optical modulator interposed therebetween.
The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawing.